<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>计时器</title>
<script type="text/javascript">
  var num=0;
  function startCount() {
    document.getElementById('count').value=num;
    num=num+1;
     setTimeout("startCount()",1000);
  }
  setTimeout("startCount()",1000);
</script>
</head>
<body>
<form>
<input type="text"  id="count" />
</form>
</body>
</html>